Information Brief - Sound Transmission

This brief covers the transmission of sound through building assemblies, such as exterior and interior walls, floors, and windows. Common sound transmission ratings are described and recommended ratings are given for various assemblies. Some common techniques and rules of thumb for reducing sound transmission are also discussed.

Changes in STC Rating Changes in Apparent Loudness
+/- 1 Almost imperceptible
+/- 3 Just perceptible
+/- 5 Clearly noticeable
+/- 10 Twice (or half) as loud

Recommended Ratings
An STC of 50 is a common building standard and blocks approximately 50 dB from transmitting through an assembly. However, occupants could still be subject to the awareness, if not understanding, of loud speech. Constructions with a higher STC should be specified in sensitive areas where sound transmission is a concern. Sound booths requiring near-perfect sound attenuation typically have STC ratings of 65.

To help illustrate the effects of increasing the STC of a wall assembly, the chart below pairs common sounds with various STC ratings.

STC What can be heard
25 Normal speech can be understood quite easily and distinctly through wall
30 Loud speech can be understood fairly well, normal speech heard but not understood
35 Loud speech audible but not intelligible
40 Onset of "privacy"
42 Loud speech audible as a murmur
45 Loud speech not audible; 90% of statistical population not annoyed
50 Very loud sounds such as musical instruments or a stereo can be faintly heard; 99% of population not annoyed.
60+ Superior soundproofing; most sounds inaudible

The Uniform Building Code (UBC) contains requirements for sound isolation for dwelling units in Group-R occupancies, including hotels, motels, apartments, and condominiums. UBC requirements for walls, ceilings, and floors are as follows: STC rating of 50 (if tested in a laboratory) or 45 (if tested in the field). The field test evaluates the dwelling's actual construction and includes all sound paths, including flanking paths, that may be present.

Sound Paths
Even with a high STC rating, any penetration or air-gap in a partition, or "flanking path" around the partition, can seriously degrade the sound isolation quality of that assembly. Flanking paths are the means for sound to transfer from one space to another without traveling directly through the assembly. Sound can flank over, under, or around a wall. Sound can also travel through common ductwork, plumbing, or corridors. Noise will travel between spaces at the weakest points. This means that the added investment in a wall with a high STC rating can easily be lost unless all the weak points are also controlled.

Windows and doors are typically the weak links in exterior walls. They generally have lower STC ratings than common exterior walls and provide flanking paths for sound around the wall. The presence of windows and doors can compromise the sound control of otherwise high-STC assemblies. However, the actual dB reduction windows and doors provide varies over a range of frequencies. At mid to high frequencies, windows and doors generally provide 10 to15 dB less sound attenuation than common exterior walls. But at low frequencies, the wall itself becomes the weak link and windows and doors generally provide 5 dB more sound attenuation. The relationship between frequency and sound attenuation illustrates why it's important to know the type of sound (high or low frequency) that needs to be blocked.

Figure 1 Weak links in a typical wall over a range of frequencies


Increasing the STC Rating

Adding Insulation in the Partition
Adding insulation or other sound-absorptive material to a partition is a low-cost method to increase its STC rating. Usually, insulation is added between structural studs or between the studs and a finish layer, such as gypsum board. Installing insulation within a wall or floor/ceiling cavity will improve the STC rating by about 4-6 dB, which is clearly noticeable.

Increasing or Adding an Air Gap in the Partition
Adding an air space within a partition can also help to increase sound attenuation. An air space effectively creates two independent walls. However, the STC rating will be much less than the sum of the STC ratings of the individual walls. A common method to add an air space is with resilient channels and another layer of gypsum board. An airspace of 1 ½" will improve the STC of a partition by approximately 3 dB, while an air space of 3" will improve the STC by approximately 6 dB. In general, the wider the air space is, the greater its effectiveness.

Adding Mass to the Partition
The mass of a partition is a key factor in its ability to block sound. For example, a heavy concrete wall will block more sound than a lightweight 2x4 wall with gypsum board. Mass is commonly added to existing walls by adding additional layers of gypsum board. When the mass of a barrier is doubled, the isolation quality (STC rating) increases by approximately 5 dB, which is clearly noticeable.

For Windows
There are several strategies to increase the sound attenuation of windows. The best option is to add an additional layer of glass, separated by an air gap of ½" or more. Air gaps less than ½" add relatively little sound attenuation. Thus, the STC rating of a triple glazed window with two ¼" air gaps is rarely higher than the STC rating of double-glazed window with a ½" air gap. However, triple-glazed windows with two ½" air gaps outperform double-glazed windows with a single ½" air gap. Other strategies for improving the STC ratings of windows include using thicker panes of glass, using panes of glass with differing thicknesses, using laminated glass, and adding additional frame seals.